Concept of Bioelectronic Nose - Bioelectronic Nose: Integration of Biotechnology and Nanotechnology - page 8

In olfactory signaling, the activated ORs induce the dissociation of the α-subunit
of G proteins. In order to utilize this event, ORs and Gα olf proteins were immobi-
lized onto the chip of SPRs and can be used to analyze the association and dissocia-
tion of ligands [ 68 , 69 ]. The activation of ORs could be monitored by measuring
the release of Gα subunits. SPRs have also been utilized for the development of
cell-based bioelectronic noses [ 75 , 78 ]. The cells were cultured on SPR chips. Then,
the inflow of ions into the cells was measured. The influx of calcium ions that oc-
curs by the olfactory signaling affects the resonance angle of the SPR chip, and the
odorants could be detected. Detailed information about the SPR-based bioelectronic
noses is described in Chap. 11.

1.2.2.3 Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) is an effective technique to char-
acterize electrodes functionalized with biomolecules. EIS technique commonly re-
quires counter, reference, and working electrodes. For bioelectronic noses, the sur-
face of the working electrode was immobilized with ORs. The binding of odorants
to ORs was measured by recoding the experimental impedance spectrum. Using this
principle, EIS-based bioelectronic noses have been successfully developed [ 86 , 87 ].

1.2.2.4 Planar Microelectrode

Planar microelectrodes have also been used to develop cell-based bioelectronic
noses [ 77 ]. The cells expressing olfactory receptors are cultured in a chip patterned
with planar microelectrodes. After treatment with specific odorants, positive ions
flow into the cells. The influx of ions is subsequently transduced into electrical
signals through planar electrodes. In addition, the signals could be amplified by
electrical stimulation [ 76 ]. These results demonstrated the possibility of developing
cell-based bioelectronic noses using planar microelectrodes.

1.2.2.5 Field-Effect Transistor

Nanomaterial-based field-effect transistors (FETs) have been used for the develop-
ment of bioelectronic noses. FET sensors react to the event of changes in charge
that occur near the sensing channels and generate highly sensitive responses. In
order to improve selectivity, FETs are generally functionalized with diverse types
of biomaterials such as OR proteins [ 45 , 46 , 70 - 72 ], nanovesicles [ 80 , 81 ], and OR-
derived peptides [ 50 ]. The binding event between OR and odorant induces changes
in charge of the ORs. This change is subsequently converted into highly sensitive
electrical signals through nanomaterials [ 45 ]. Nanomaterials with semiconduct-
ing properties have often been utilized to fabricate the sensing channels of FETs,
such as single-walled carbon nanotubes (CNTs), conducting polymer nanotubes